I'm fairly sure just talking to them at lunch or break does the trick.
was briefly kinda stuck in similar place
convinced the first few, but it says Find Another Girl and I couldn't figure out who I'd missed for a while
turned out it was Nissa (blue hair) in the yard
this eventually led to my favorite (so far) scene in the game, the shower pic where the red-haired girl tells Jess she'll enjoy some of the things Karen does to Missy :)